Mesozooplankton Grazing and Egg Production of the Copepod Calanus pacificus in Relation to Evolution of Newly-Upwelled Filaments off Central California

We quantified mesozooplankton grazing, as well as egg production rates of the copepod Calanus pacificus, throughout the evolution and offshore transport of two newly-upwelled water filaments off central California in June 2017 and August 2019. Grazing was assessed in 5 mesozooplankton size fractions, using the gut fluorescence method. Day/night differences in grazing were most pronounced in the largest size fractions. Spatial changes in grazing impact of different mesozooplankton size fractions will be assessed in relation to filament age and phytoplankton growth rates. We observed highest egg production rates for Calanus pacificus in 1-2-week-old filament waters in Aug 2019 (average 50 eggs/female/day). Newly-upwelled (<1 week old) waters produced average rates of 35-40 eggs/female/day across both years. Offshore oligotrophic (non-filament) waters produced near-zero egg production despite visibly robust adult females with largest body lengths. Recently-upwelled filaments are likely essential habitat for maximal reproductive success of dominant California Current System copepod species such as C. pacificus. However, hatching success of eggs (to nauplius) was somewhat lower in nearshore than offshore waters across both years. Egg production rates show an Ivlev relationship with total chlorophyll across both the 2015-16 El Niño and non-Niño conditions, suggesting that food availability is a dominant determinant of reproductive success regardless of large-scale interannual ocean changes.